I understand that as an (new)Elite member I get Preffered seating?

How early in advance am I allowed to pick seats in the Y class cabin on an international flight?

I have booked a flight for April (AC 1 and AC 2) on an A340 and I am trying to get bulkhead seat 12A on both flights.

Does anyone know when I will be allowed to pick out seats? or if I would be allowed to get anytime soon as an Elite?

Also, I have 2 segments on United for this trip. Does anyone know their policies regarding seat assignments in Y for AP Elite members?

Thanks,
Jason